The coupling of the exergonic ATP hydrolysis reaction to the endergonic glutamine synthesis reaction. 
In thermodynamics, free energycoupling refers to the rule that the free energy changes of coupled reactions are additive. Specifically, given a set of reactions, i through k, typically occurring under isothermal-isobaric (biological) conditions, the total Gibbs free energy change, for the mixture of reactions will be the summation of the free energies of the component reactions:
G = U + PV – TS
In other words, the energy released from one reaction (spontaneous ones) will, in effect, drive other reactions which are not energetically favored (non-spontaneous ones).
In short, given two or more reactions in which one is not energetically favorable the logic of free energy coupling, which seems to be based on the rule that state functions are additive, stipulates that the set of coupled or energetically connected reactions in contact will proceed, when mixed together, as long the magnitude of the free energy change for the exergonic reaction[s] is greater than that of the endergonic reaction[s], whereby the free energy released from one or more powerful reactions will drive the weaker free energy absorbing reactions.
If, for example, the one spontaneous reaction is mixed with a non-spontaneous reaction, such as shown below:
A → B
C → D
the non-spontaneous reaction can be driven into reaction as long as: |ΔGAB| > |ΔGCD|.
Further reading ● Delbruck, Max. (1944). “Problems of Modern Biology in Relation to Atomic Physics: Part III: Energy-Coupling”, A Series of Lectures, April and May, Vanderbilt University School of Medicine. ● Andrews, Andres J. and Luger, Karolin. (2010). “Chapter eleven: A Coupled Equilibrium Approach to Study Nucleosome Thermodynamics” (abs), Methods in Enzymology, 488: 265-85.
External links ● Bioenergetics (see: Energy Coupling), Biochemistry and Biophysics Program at Rensselear Polytechnic Institute – rpi.edu.